Histone deacetylases (HDACs) are a class of enzymes that catalyzes the removal of an acetyl group from an α-N-acetyl lysine amino acid residue from other proteins, mainly histones. The histones are an essential part of how the genome is stored in the cell nucleus and DNA expression is regulated by histone acetylation and de-acetylation. Lysine acetylation is a key post-translational modification of many proteins, and which underlie many aspects of gene transcription, cellular signaling, cellular transport and metabolic changes (Kouzarides et al. 2007, Choudhary et al. 2009, Zhao et al. 2010). HDACs have pivotal roles in the regulation of gene expression, forming complexes with DNA binding proteins and thereby affecting histone acetylation and chromatin accessibility at promoter regions. These enzymes also have non-histone substrates, such as transcription factors and structural proteins whose biological activity is partly regulated by acetylation.
The common classification of human deacetylases is based on molecular phylogenetic analysis of primary structure, subsequently grouped based on homology to yeast enzymes (Gregoretti et al. 2004). This approach yields four distinct classes that vary in size and function. Class I (HDAC1, HDAC2, HDAC3 and HDAC8), class IIa (HDAC4, HDAC5, HDAC7 and HDAC9), class IIb (HDAC6 and HDAC10) and class IV (HDAC11). The HDACs require a divalent ion for catalysis. The class III proteins form a structurally and mechanistically distinct class of hydrolases dependent on nicotinamide adenine dinucleotide (NAD+) (sirtuins, Sirt1-Sirt7) (Smith et al. 2008). The class I HDACs are found primarily in the nucleus, while the class IIa and class IIb HDACs are able to translocate in and out of the nucleus, depending on different signals.
There are numerous diseases that are related to dysregulated HDAC enzymatic function, including cancer, autoimmune and neurodegenerative disorders (Karberg 2009). For example, overexpression of specific HDACs has been identified in a range of human cancers, including HDAC1 in gastric and prostate cancer, HDAC1 and HDAC6 in breast cancer, and HDAC2 and HDAC3 in colorectal cancer (Ververis et al. 2013). Extensive cell-based assays and clinical studies with HDAC inhibitors have been shown to reduce proliferation, induce cell death and apoptosis, cause cell-cycle arrest, and prevent differentiation and migration selectively in malignant and transformed cells with little effect in normal cells (Ververis et al. 2013). Thus, HDAC inhibitors have the potential to be used as mono-therapies in oncology. In addition to their intrinsic cytotoxic properties when tested as a single treatment, HDAC inhibitors have been shown to induce additive cytotoxic effects when used in combination with conventional anticancer therapies, such as chemotherapy (anthracyclines and retinoic acid) and radiotherapy. Furthermore, studies with HDAC inhibitors in combination with ultraviolet radiation and potent iodinated DNA minor groove-binding ligands have been shown to augment photosensitization and cytotoxicity in tumor (Ververis et al. 2013). Currently (2015), there are five HDAC inhibitors that have received approval from the US FDA for the treatment of various cancers: vorinostat (suberoylanilide hydroxamic acid, Zolinza), depsipeptide (romidepsin, Istodax), belinostat (PXD101, Beleodaq), pracinostat (SB939), and panobinostat (LBH-539, Farydak). Many clinical trials assessing the effects of various HDAC inhibitors on hematological and solid cancers are being conducted (Ververis et al. 2013). The five approved inhibitors are active against several members of the HDAC family of enzymes leading to acute toxicities such as gastrointestinal symptoms and myelosuppression as well as severe fatigue (Prince et al. 2009). Also, the risk of significant negative impact on cardiac function is considered to be large (Brana & Tabernero 2010). Several reports show that there are intrinsic toxic side effects associated with inhibition of the HDAC class I isoforms and that this prevents the application of broad spectrum and class I selective inhibitors to areas outside of oncology because of a small therapeutic window. Early clinical trials with the selective HDAC6 inhibitor ACY-1215 appear to largely circumvent undesirable side-effects classically reported with broad-acting or class I-selective inhibitors (Raje et al, 2013). Although it remains to be demonstrated in the clinic, compounds that target specific HDACs with greater selectivity may be beneficial in certain cancers (Balasubramanian et al. 2009). For example, the selective HDAC8 inhibitor PCI-3405, was shown to selectively inhibit HDAC8 and induce apoptosis specifically in T-cell lymphomas and not other tumor or normal cells, showing that HDC8 plays an important role in the pathophysiology of this disease and suggesting that therapy with an HDAC8 specific inhibitor may lead to less side effects (Balasubramanian et al. 2008).
The class IIb enzymes, HDAC6 and HDAC10, differ from the other HDACs in that they primarily localize to the cytoplasm and differ structurally by containing two catalytic sites. HDAC6 is a microtubule-associated enzyme and deacetylases primarily non-histone proteins such as α-tubulin, cortactin, and Hsp90 (Aldana-Masangkay & Sakamoto 2011). α-tubulin is involved in cytoskeletal structural integrity and cellular motility, cortactin plays a role in cell motility, while Hsp90 (heat shock protein) is a molecular chaperone helping client proteins to fold properly and maintain function. The therapeutic areas most susceptible to alterations in HDAC6 activity appear to be cancer, autoimmune disorders, and neurodegenerative diseases. In contrast to other HDACs and especially class I isoforms, the loss of function of HDAC6 does not produce toxicity or major developmental defects in rodents (Govindarajan et al. 2013; Morris et al. 2010; Zhang et al. 2008). Inhibition of HDAC6 does not appear to be associated with the same level of toxicity observed with inhibition of the class I isoforms. The lower level of toxicity associated with HDAC6 inhibition compared to inhibition of the HDAC class I isoforms suggest that selective inhibition may provide a way to circumvent toxicity issues and thereby allow a superior side-effect profile and/or a higher dose with an accompanying superior effect on target. This may permit treatment of a wider range of cancer diseases and also treatment of non-oncology diseases requiring a wider therapeutic window (Best & Carey 2010, Zhang et al. 2008).
Cancer
Oncogenes, such as Ras, deregulate fundamental cellular functions, which can lead to the development of tumors and metastases. The Ras/MAPK signaling pathway is known to be required for tumorigenesis and HDAC6 is required for Ras-induced oncogenic transformation by providing anchorage-independent proliferation (Aldana-Masangkay & Sakamoto 2011). This allows the cancer cell to divide freely without being part of a tissue and is a hallmark of malignant transformation. Further, it has been shown that HDAC6 is required for oncogenes to be able to change the spatial organization of the vimentin fibers of the intracellular cytoskeleton which will induce cell stiffness and promote the invasive capacity of cells (Rathje et al. 2014). Thus, HDAC6 activity contributes to cell changes that lead to both tumor formation and invasion of tumor cells into healthy tissue (metastases).
The antitumor effect observed via HDAC6 inhibition is probably the result of multiple mechanisms involving cell motility/migration, invasion, angiogenesis, induction of apoptosis, and inhibition of DNA repair (Kalin & Bergman 2013). HDAC6 knockout mice demonstrated reduced phosphorylation of AKT and ERK1/2 (signaling pathways involved in tumor growth) and lower levels of activated Ras than those derived from wild-type mice (Lee et al. 2008). HDAC6 knock-down cells from SCID mice subcutaneously injected with HDAC6 specific shRNA showed retarded growth. By reconstitution with wild type HDAC6, but not with catalytically inactive mutant HDAC6, these knock-down cells regained its phenotype indicating that HDAC6 is specifically required for tumorigenic growth (Lee et al. 2008). Another method to combat cancer cells is to target the two major pathways for protein turnover in eukaryotic cells—the Ubiquitin-Proteasome-System (UPS) and the HDAC6-dependent lysosomal pathway. HDAC6 directly interacts with misfolded or poly-ubiquinated proteins to target them for lysosome-mediated protein degradation via aggresome formation and autophagy (Aldana-Masangkay & Sakamoto 2011). If UPS activity is insufficient, this HDAC6 dependent pathway is able to compensate for intracellular protein degradation. Cancer cells accumulate more misfolded proteins compared to nonmalignant cells and depend on efficient disposal of these misfolded proteins for cell survival. Thus, simultaneous inhibition of proteasome and HDAC6 activities has been proposed as a strategy to synergistically induce cancer cell death. Successful examples of this approach have used the proteasome inhibitor bortezomib together with different specific HDAC6 inhibitors such as tubacin on multiple myeloma cells (Hideshima et al. 2005), NK84 on ovarian cancer cells (Bazzaro et al. 2008), and ACY-1215 on cells and animal models of multiple myeloma (Santo et al., 2012). In all cases the two inhibitors showed synergistic effects and high selectivity for cancer cells compared to normal cells.
Autoimmune Disorders
There is strong evidence supporting HDAC6 as a target for the treatment of numerous autoimmune disorders (Greer et al. 2012). In murine models, pan-HDAC inhibitors, such as vorinostat and TSA, were able to alleviate the symptoms and reverse the progression of established colitis (de Zoeten et al. 2011). HDAC6 selective inhibitors such as tubacin and tubastatin A but not class I selective HDAC inhibitors such as entinostat were able to confer protection in these in vivo models. In murine models of allograft rejection tubacin and tubastatin A in combination with low-dose rapamycin, a clinically used immunosuppressant, were able to significantly increase the lifespan of mice from approximately 15 days to more than 60 days in comparison to mice treated with rapamycin alone (de Zoeten et al. 2011). This combination therapy was only administered for 14 days but was able to confer long term protection against allograft rejection.
Mental Disorders
In the mammalian brain, HDAC6 is mainly found in neurons (Southwood et al., 2007) and with the highest levels at the dorsal and median raphe nuclei, parts of the brain that are involved in emotional behaviors. HDAC6-deficient mice exhibit antidepressant-like behavior in behavioral tests, and this was mimicked by administration of NCT-14b, a HDAC6-specific inhibitor, to wild type mice (Fukada et al., 2012). Further, selective knockout of the highly abundant HDAC6 in serotonin neurons reduced acute anxiety caused by administration of the steroid hormone corticosterone, and blocked the expression of social deficits in mice exposed to inescapable traumatic stress (Espallergues et al., 2012). Administration of the selective HDAC6 inhibitors ACY-738 and ACY-775 has been shown to induce dramatic increases in α-tubulin acetylation in brain and stimulate mouse exploratory behaviors in novel, but not familiar environments (Jochems et al. 2014). The two compounds share the antidepressant-like properties of pan-HDAC inhibitors, such as SAHA and MS-275, in the tail suspension test and social defeat paradigm without any detectable effect on histone acetylation. These effects of ACY-738 and ACY-775 are directly attributable to the inhibition of HDAC6 expressed centrally, as they are fully abrogated in mice with a neural-specific loss of function of HDAC6. Taken together, these findings suggest that HDAC6-mediated reversible acetylation contribute to maintain proper neuronal activity in serotonergic neurons, and also provide a new therapeutic target for depression. In addition, acute stress, via glucocorticoid receptors (GRs), enhances glutamatergic signalling in the prefrontal cortex, a region responsible for high-order cognitive functions. It has been shown (Lee et al. 2012) that inhibition or knockdown of HDAC6 blocks the enhancement of glutamatergic signalling by acute stress and that inhibition or knockdown of the GR chaperone protein Hsp90 (a HDAC6 substrate) produces a similar blockade of the acute stress-induced enhancement of glutamatergic signalling. This suggests that HDAC6 is a key controller of neuronal adaptations to acute stress and that inhibition of HDAC6 may provide neuroprotective effects against stress-induced mental illness.
Neurodegenerative Disorders
There are numerous reports suggesting that HDAC6 inhibition exert neuroprotection which may benefit patients afflicted with neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases as well as patients afflicted by traumatic brain injury (TBI) and inherited neurological disorders such as Charcot-Marie-Tooth disease (CMT) and Rett syndrome (Kalin & Bergman 2013, Simoes-Pires et al. 2013). On the other hand, an induction of HDAC6 would theoretically contribute to the degradation of protein aggregates which characterize various neurodegenerative disorders (Simoes-Pires et al. 2013). HDAC6 has been identified as a potential therapeutic target to modulate Alzheimer's disease (AD) pathogenesis. Specific HDAC6 inhibitors exert neuroprotection by increasing the acetylation levels of α-tubulin with subsequent improvement of the axonal transport, which is usually impaired in neurodegenerative disorders such as AD (Simoes-Pires et al. 2013). The loss of proper axonal transport leads to synaptic degradation through impaired mitochondrial and neurotransmitter trafficking (Kalin & Bergman 2013). It has been demonstrated that treatment of neurons with amyloid beta (Aβ) oligomers significantly attenuated mitochondrial elongation and transport, which was subsequently alleviated by treatment with the HDAC6 inhibitor tubastatin A (Kim et al. 2012). In another report, it was shown that reducing endogenous HDAC6 levels in an AD mouse model restored learning and memory (Govindarajan et al. 2013). These results suggest that HDAC6 inhibition may slow or reverse the neuronal damage associated with Aβ and thus represents a viable drug target for the treatment of AD. Further, HDAC6 together with Hsp90 and the ubiquitin ligase CHIP form a network of chaperone complexes that modulates levels of tau—the microtubule-associated protein that is hyperphosphorylated and forms the pathological hallmark of neurofibrillary tangles in AD (Cook & Petrucelli 2013). It has been demonstrated that HDAC6 levels positively correlate with tau burden, while a decrease in HDAC6 activity or expression promotes tau clearance (Cook et al., 2012). Inhibition or depletion of HDAC6 causes Hsp90 hyperacetylation and the concomitant decreased affinity of Hsp90 for client proteins such as tau, leads to client protein degradation (Kalin & Bergman 2013). In addition, loss of HDAC6 activity augments the efficacy of an Hsp90 inhibitor, opening the possibility to synergistically promoting the degradation of Hsp90 client proteins by co-treatments with both HDAC6 and Hsp90 inhibitors, as has been shown for leukemia cells (Cook et al. 2012; Rao et al. 2008; George et al. 2005).
The neuroprotective effect of HDAC6 inhibition may be beneficial for patients suffering from traumatic brain injuries. For example, it has been reported that HDAC6 inhibition results in the hyperacetylation of peroxiredoxin-1 and -2 leading to increased resistance against oxidative stress such as that observed during ischemic stroke (Parmigiani et al. 2008). HDAC6 inhibition may also be beneficial for patients afflicted by inherited neurological disorders such as Charcot-Marie-Tooth disease (CMT) and Rett syndrome. For example, symptomatic improvement was observed in a transgenic mouse model of CMT after the treatment with specific HDAC6 inhibitors, together with the increase in tubulin acetylation (D'Ydewalle et al. 2011). HDAC6 inhibition by tubastatin A has been shown to restore brain-derived neurotropic factor (BDNF) neurological function in Mecp2 knockout hippocampal neurons showing that HDAC6 is a potential target for Rett syndrome (Xu et al. 2014).
The above described data serve to illustrate the validity of modulating HDAC6 activity for treatment of disorders and diseases that include not only hyperproliferative indications, such as cancer, but also other therapeutic areas such as neurodegenerative disorders, autoimmune disorders, and mental disorders.